1. Field of the Invention
The present invention relates to a zoom lens having fluctuation prevention function and a photographing apparatus provided with the zoom lens. In particular, the present invention relates to a zoom lens having fluctuation prevention function and a photographing apparatus, which can be preferably applied to a still camera, a video camera, a digital camera, etc., and which is designed to allow a lens group, i.e., a part of the zoom lens, to be moved in a direction perpendicular to an optical axis so as to optically correct a blur of a photographed still image caused by the fluctuating motion (inclined motion) of the zoom lens due to hand fluctuation or the like, thereby obtaining a stabilized photographed image.
2. Related Background Art
Blur is caused on a photographed image due to hand fluctuation transmitted to a photographing system. Various attempts have been made to provide fluctuation preventive optical systems that can prevent the blur on the photographed image.
Recently, for a still camera, a video camera, a digital camera, etc., a zoom lens having fluctuation prevention function has been required to correct the image blur caused due to hand fluctuation or the like for the purpose of obtaining a higher quality image and enlarging applicable photographing conditions.
As a zoom lens having fluctuation prevention function, for example, Japanese Patent Application Laid-Open No. 9-230236 discloses a four-lens-group zoom lens in which positive, negative, positive and positive refractive power lens groups are arranged in this order from an object side, and in which the third lens group is divided into a positive refractive power front lens group and a positive refractive power rear lens group, and the rear lens group is moved perpendicularly to an optical axis to correct the image blur. Japanese Patent Application Laid-Open No. 10-232420 discloses a four-lens-group zoom lens, primarily applied to a video camera, in which positive, negative, positive and positive refractive power lens groups are arranged with the first and third lens groups to be fixed, and in which the third lens group is divided into a positive refractive power lens group and a negative refractive power lens group, and either one of these lens groups is moved in a direction perpendicular to an optical axis to correct image blur.
Japanese Patent Application Laid-Open No. 7-128619 discloses a magnification changeable optical system having four lens groups, which is provided, sequentially from an object side, with a first group having a positive refractive power fixed during the change of magnification and the in-focusing condition, a second group with a negative refractive power, having a magnification changing function, an aperture diaphragm, a third group with a positive refractive power, and a fourth group with a positive refractive power, that has both correction a function for correcting an image plane that varies depending on the change of magnification, and an in-focusing function. In the system, the third group is constructed of two lens groups, i.e., a third-1 group with a negative refractive power, and a third-2 group with a positive refractive power, and the third-2 group is moved in a direction perpendicular to an optical axis to correct blur of the photographed image caused by fluctuation of the magnification changeable optical system.
Japanese Patent Application Laid-Open No. 7-199124 discloses a magnification changeable optical system constructed of four groups, i.e., positive, negative, positive and positive refractive power lens groups, in which the third lens group entirely is fluctuated perpendicularly to an optical axis for the purpose of fluctuation prevention.
In general, an optical system, which is designed to parallell offset a part of lens of a photographing system in a direction perpendicular to an optical axis to correct the image blur, can advantageously realize the correction of the image blur in a relatively simple fashion, but requires driving means for driving the lens to be moved. Further, the system provides the result in an increased amount of offset aberration occurring during fluctuation prevention.
For example, if a correction lens group for correcting the image blur is large in its number of constituting lenses and weight, a large torque is required when electrically driving the same. Further, if the correction lens group for correcting the image blur is set inappropriately, the correction optical system requires a large amount of its movement in order to obtain an image-blur correcting effect of a certain degree, making the entire optical system large in size.
An object of the present invention is to provide a zoom lens and a photographing apparatus using the zoom lens, in which a relatively small and light-weight lens group forming a part of the zoom lens is moved in a direction perpendicular to an optical axis to correct image blur due to fluctuation (inclination) of the zoom lens, and in which the lens arrangement for the zoom lens is optimized to realize a zoom lens that is compact in size, has a simple mechanism, is small in the load applied to driving means, and that can effectively correct certain image blurring with a small amount of displacement of the lens group.
To attain the above-noted object, the present invention provides, as one form thereof, a zoom lens which includes:
a first lens unit with a positive refractive power;
a second lens unit with a negative refractive power;
a third lens unit with a positive refractive power; and
a fourth lens unit with a positive refractive power,
in which at least the first, third and fourth lens units are moved along an optical axis to vary spatial distances of the lens units to thereby the change magnification,
and the third lens unit includes a first lens subunit with a positive refractive power, and a second lens subunit with a negative refractive power, and the second lens subunit is moved to have at least a perpendicular vector component with respect to the optical axis to thereby displace an image.
As a preferable form, the following conditions are satisfied:
0.35 less than F3/Fm less than 1
xe2x88x920.9 less than F3/F32 less than xe2x88x920.18
where focal lengths of the entire system at a wide angle end and a telephoto end are respectively represented by Fw and Ft, the focal length of the third lens unit is represented by F3, the focal length of the second lens subunit is represented by F32, and Fm=(Fwxc3x97Ft)xc2xd.
As a preferable form, the second lens subunit is moved to have a perpendicular vector component with respect to the optical axis to thereby correct image blur caused by fluctuation of the zoom lens.
As a preferable form, the following condition is satisfied:
xe2x88x920.2 less than (Ra+Rb)/(Raxe2x88x92Rb) less than 0.7
where the radius of curvature of a lens plane located closest to an image plane side within the first lens subunit is represented by Ra, and the radius of curvature of a lens plane located closest to the object side within the second lens subunit is represented by Rb.
As a preferable form, the following conditions are satisfied:
0.7 less than F1/Fm less than 2.8
0.15 less than |F2/Fm| less than 0.7
0.5 less than F4/Fm less than 2.0
where the focal length of an i-th lens unit of the lens units is represented by Fi.
As a preferable form, the second lens subunit includes one positive lens and one negative lens.
As a preferable form, the first lens subunit includes, from the object side, a composite lens with a positive refractive power, in which a meniscus-like negative lens having a concave surface on an image plane side is adhered to a positive lens, and a composite lens with a positive single lens or a positive refractive power, in which a positive lens is adhered to a negative lens.
As a preferable form, the second lens subunit includes a composite lens with a negative refractive power, in which a positive lens having a convex surface oriented to an image plane side is adhered to a negative lens having a concave surface oriented to the object side.
As a preferable form, the third lens unit further includes a third lens subunit having a negative or positive refractive power on an image plane side of the second lens subunit.
To attain the above-noted object, the present invention provides, as another form, a photographing apparatus which includes:
a first lens unit with a positive refractive power;
a second lens unit with a negative refractive power;
a third lens unit with a positive refractive power;
a fourth lens unit with a positive refractive power; and
a casing holding a zoom lens,
in which at least the first, third and fourth lens units are moved along an optical axis to vary spatial distances of the lens units to thereby change the magnification,
and the third lens unit includes a first lens subunit with a positive refractive power, and a second lens subunit with a negative refractive power, and the second lens subunit is moved to have at least a perpendicular vector component with respect to the optical axis to thereby displace an image.
As a preferable form, the following conditions are satisfied:
0.35 less than F3/Fm less than 1
xe2x88x920.9 less than F3/F32 less than xe2x88x920.18
where the focal lengths of the entire system at a wide angle end and a telephoto end are respectively represented by Fw and Ft, the focal length of the third lens unit is represented by F3, the focal length of the second lens subunit is represented by F32, and Fm=(Fwxc3x97Ft)xc2xd.
As a preferable form, the second lens subunit is moved to have a perpendicular vector component with respect to the optical axis to thereby correct image blur caused due to fluctuation of the zoom lens.
As a preferable form, the following condition is satisfied:
xe2x88x920.2 less than (Ra+Rb)/(Raxe2x88x92Rb) less than 0.7
where the radius of curvature of a lens plane located closest to an image plane side within the first lens subunit is represented by Ra, and the radius of curvature of a lens plane located closest to the object side within the second lens subunit is represented by Rb.
As a preferable form, the following conditions are satisfied:
0.7 less than F1/Fm less than 2.8
0.15 less than |F2/Fm| less than 0.7
0.5 less than F4/Fm less than 2.0
where the focal length of an i-th lens unit of the lens units is represented by Fi.